In Vivo Analysis of Viral Cyclin: Infection by herpesviruses causes an acute lytic infection that is rapidly cleared, and subsequent establishment of a latent infection that exists in balance with the host immune system for the lifetime of a healthy host. The gammaherpesviruses include the human pathogens, Epstein Barr virus and Kaposi's sarcoma herpesvirus, and the murine gammaherpesvirus-68 (gHV68). These viruses are related by sequence conservation, genomic organization, tropism, and associated pathologies. Latently infected cells express a limited set of viral genes, yet remain capable of reactivating lytic infection upon stimulation. Latency and reactivation from latency are associated with chronic disease and malignancies, particularly in immunocompromised hosts. Regulation of viral infection is a complex relationship between host and viral factors. These processes, from initial infection to latency and reactivation, are not well understood and have been constrained by the difficulties of studying higher mammalian host systems. The murine gHV68 model system provides a tractable small animal model in which both pathogen and host are genetically manipulable, and enables study of virus and host interactions through the entire course of viral infection. Previous utilization of gHV68 as a model system to study cell cycle regulation in viral infection demonstrated that the gHV68 viral cyclin promotes cell cycle progression and acts as an oncogene when expressed as a transgene in primary lymphocytes. The gHV68 viral cyclin is not required for any aspect of lytic infection, but is essential for efficient reactivation from latency. This proposal will address the mechanism of action of the gHV68 viral cyclin in infection. First, what is the expression pattern of the viral cyclin in infection? Second, what are the catalytic partners and the substrates of the viral cyclins in vivo? Third, are the unique features of the viral cyclins essential to their role in viral infection?